1. Field of the Invention
This invention relates to masks for lithography and more particularly to phase-shift masks for use in photolithography.
2. Description of Related Art
In photolithography, masks are employed to expose a pattern upon a work piece. As manufacturing requirements call for exposure of patterns with smaller and smaller dimensions, it is becoming necessary to employ techniques which permit enhancement of the current performance of the process of photolithography. One trend has been to use electromagnetic energy with shorter wavelengths in the UV wavelengths, x-rays and the like. An alternative approach is to use phase-shifting techniques in the ranges of wavelengths used in photolithography in the past.
The phase-shifting technique can improve the resolution of coherent or partially coherent optical imaging systems. It has been shown that the normalized resolution k.sub.1 can be reduced from 0.7 to 0.35 to improve lithography by two generations.
Use of phase shifting techniques with masks in optical microlithography systems has been shown to improve projected images. These phase-shifting techniques have been practiced in various configurations.
FIGS. 1A and 1B illustrate the alternative-element (Alt) phase-shifting method which involves shifting every other one of the adjacent transparent elements. FIG. 1A shows a binary intensity mask (BIM) and FIG. 1B shows a phase-shifting mask (PSM). In particular, in both FIG. 1A and FIG. 1B the glass 10 is coated with an array of chrome elements 12 separated by spaces 11 and 13. In FIG. 1B, a phase-shifter layer 14 is deposited in space 13 between two adjacent ones of the elements 12 upon the exposed glass 10 leaving an exposed space 11 in the other space between the adjacent elements 12. It can be seen that the value of E, the amplitude of the electric field of the electromagnetic radiation, at both the mask and the wafer is reversed to a negative value beneath the phase-shifter layer 14 in FIG. 1B from that in the corresponding position in FIG. 1A. In FIG. 1B, the intensity I of portions of the curves at 11''' and 13''' at the wafer is significantly changed in that the contrast is enhanced beneath the two spaces 11 and 13. The values of the electric field E at the mask 11' and 13' are beneath the spaces 11 and 13 as are the values 11" and 13" for the electric field E at the wafer.
Another prior art design, called Subresolution-assisted (SA) PSM involves addition of phase-shifted subresolution elements to isolated transparent features as shown in FIGS. 2A and 2B. Both techniques suffer from not being applicable to an arbitrary mask layout which consists of many features other than closely packed or isolated transparent features. FIG. 3A shows a substrate 30 carrying an absorber 31 carrying a phase-shifter 32 beyond the absorber 31. We refer below to the technique employed here using a projection beyond the absorber as the rim PSM technique. If the absorber 31 were not used, phase shifter light of a large negative amplitude creates unwanted bright images. The absorber 31 blocks the unwanted part of the phase-shifted light to eliminate the unwanted bright spots at the center of the phase-shifted features. On the right side of FIG. 3A, a conventional absorber 33 is shown adjacent to the absorber 31. FIGS. 3B and 3C are aligned with FIG. 3A to show the relative results of the differences in the masks on the left and right. FIG. 3B shows the electric field E at the mask for the structure of FIG. 3A.
FIG. 3C shows the intensity at the wafer for the structure of FIG. 3A. Curve 34 shows what the intensity I would be without absorber 31. Curve 35 shows how it is reduced below blocker 31, with no change elsewhere.
This rim PSM technique applies to all features on any mask. However, optical proximity effects are enhanced. That is, the different features now require a larger difference in exposure to print the same feature size with an identical tolerance, as shown in the Exposure-Defocus (E-D) diagrams of k.sub.2 as a function of exposure dosage drawn in log scale in FIG. 5 which relate to teachings in an article by Burn J. Lin, "Partially Coherent Imaging in Two Dimensions and the Limits of Projection Printing in Microfabrication", IEEE Transactions on Electron Devices, Vol. ED-27, pp. 931-938 (1980) and Burn Jeng Lin, "A Comparison of Projection and Proximity Printings--From UV to X-ray" Microelectronic Engineering Vol. 11, (1990), pp. 137-145.
Yet another prior art design is shown in FIG. 4 where there is no absorber. Only the phase shifter 41, 42, 43 on substrate 40 carry the burden of patterning. The large phase shifter areas 42 are printed everywhere inside and outside the features except at the edge, where due to the large phase transition, large dark line images 42''' are produced. In the small areas 41, the edges are sufficiently close to each other so that a completely dark feature 41''' is created. Large dark images 43''' can be produced by grouping many subresolution phase shifter features 43 closely together. Here, because the phase shifters are completely transparent as opposed to the attenuated PSM to be described in the invention, this particular PSM system is called unattenuated (Utt) PSM.
The problem solved by this invention is that poor image contrast and shallow depth of focus result when steppers ar used beyond their limits.